1. Field of the Invention
The present invention relates to ATM networks, particularly IPATM networks adapted to provide multipoint-to-multipoint multi-casting, a method of multipoint-to-multipoint multi-casting in an IPATM network and telecommunications system incorporating IPATM networks adapted to provide multipoint-to-multipoint multi-casting.
2. Discussion of Background
One of the main characteristics of multi-casting in the Internet is that, see S. E. Deering, “Multi-cast Routing in Internetworks and Extended LANs,” in Proc. ACM SIGCOMM '88. August 1988, it is inherently multipoint-to-multipoint (mpt-to-mpt). However, ATM does not directly support mpt-to-mpt. This means that if ATM is to be used as an underlying technology for transport of IP multi-cast packets, efficient mpt-to-mpt communication support is essential. This can be achieved in the following ways:                (1) set-up a virtual circuit between every sender and receiver;        (2) set-up N point-to-multipoint connections; or        (3) alternatively, by providing mpt-to-mpt connections at the ATM level, as proposed by the present invention.        
Option (1) is not acceptable because it requires O(n2) connections (VCs).
Option (2) is much better than option (1) because it reduces the number of VCs to O(n). However, it still requires a complicated overlay architecture for resolving and keeping track of IP group addresses and their corresponding ATM addresses.
With option (3), only one VC is used per group and there is no need for an address repository. For details of the drawbacks found with current proposals for multi-casting in IPATM networks, see:    L. Moy, “Multi-cast Extensions to OSPF”, in Request For Comments 1584, March 1994;    G. J. Armitage, “Multi-cast and Multiprotocol Support for ATM based Internets”, ACM Sigcomm Computer Communication Rev., vol 25, April 1995;    T. Ballardie, “Core Based Tree”, SIGCOMM '93, pg: 85–95; and    D. Estrin, D. Farinacci, A Helmy, V. Jacobson, and L. Wet “Protocol Independent Multi-cast-Dense Mode (PIM-DM): Proposed experimental RFC, September 1996.
The drawbacks of current proposals for multicasting in IPATM systems can be summarized as follows:
For source routed pt-to-mpt configurations (VC mesh):                O(n2) connections are required, which do not scale for large networks;        senders must be aware of all possible receivers so they can join to the group including the receivers;        receivers must be aware of current senders;        a significant period of time is required to propagate changes whenever senders, or receivers, join, or leave, a group;        it is difficult to keep coherent and consistent group address membership lists; and        there is a high signalling overhead for joining and leaving a group;        
For server based models:                the number of VCs is reduced at the expense of increased delays;        there is a single point of failure;        traffic is concentrated;        packet reflection problems occur; and        it is difficult to keep coherent and consistent group address membership lists.        
SSAM (Simple and Scalable IPATM Multicast), the network architecture proposed for the present invention, uses a single shared tree for all senders and receivers. The tree is rooted at a core. Join requests from receivers and senders propagate towards the core. A network service, MNS (Multi-cast Network Service) provides the ATM address of the core, given an IP multi-cast address. Another important feature of this mechanism is that it provides a solution for VC interleaving problems.
In an ATM switch, when multiple incoming VCs on different switch ports must be mapped to a single outgoing VC, there is a potential contention leading to packet contention which in turn can result in packet corruption. The algorithm employed by the present invention makes an ATM switch behave as a store and forward unit, i.e. an AAL5 forwarder, or IP router, in the presence of contention, and as a cell switch in the absence of contention.
In multi-cast routing protocols using Core Based Trees (CBT), e.g. PIM (Protocol Independent Multi-casting-Sparse Mode), advertising the location of the core to every multi-cast router is an unresolved problem. Existing solutions are neither sufficiently scalable, nor sufficiently flexible.